Cutoff transmission and or reception antenna

ABSTRACT

An electromagnetic wave transmission and/or reception antenna ( 10 ) which includes a flat spiral wire, the spiral having at least two turns; the antenna including at least one cut ( 12 ) for the purpose of reducing the inter-turn capacitance. Such an antenna is used in a contactless communication system in which the reader transmits electromagnetic signals to a portable object (card or ticket) in order to identify the holder of the portable object when the latter transmits return identification signals to the reader.

The present invention broadly concerns spiral type electromagnetictransmission and/or reception antennas and particularly a spiraltransmission and/or reception antenna with cuts.

In applications where it is necessary to use transmission/receptionantennas which exchange electromagnetic waves with a portable objectpossessed by a user, it is increasingly necessary to provide relativelylarge antennas to be able to adapt to the portable object's operatingvolume. Contactless communication technology is such that the user'sportable object is a card or a ticket featuring an antenna designed toreceive electromagnetic signals sent from a reader and to transmit otherelectromagnetic signals to the reader in order to gain access to acontrolled access zone. The electromagnetic signals allow communicationnot only between the reader and the portable object but also remotepower feeding of the portable object through the physical phenomenon ofmagnetic induction.

There is a trend to increase the portable object's operating volume inorder to facilitate the passage of users who no longer have to target aspecific zone and also in order to detect the portable object held bythe user more easily (in a pocket, for example) for the general purposeof detecting fraudulent activity and/or monitor entries/exits (as in thecase of a hands-free passageway). This increase in operating volumeresults in an increase in the dimensions of the transmitter antenna andan increase in the operating distance between the transmitting antennaand the portable object. The increase in operating distance may beensured by increasing the power supplied to the antenna but this wouldinvolve an increase in electrical consumption as well as the number ofturns. The radiated magnetic field is proportional to the number ofturns when the same current runs through them.

However, the increase in the number of turns thus involves a parallelinter-turn capacitance due to the capacitive coupling between twoparallel turns of the antenna. At a given operating frequency, thehigher the capacitance, the weaker the impedance. As a result, asignificant portion of the current is dissipated by this capacitanceinstead of entering the antenna. Furthermore, interference due tocapacitive coupling between the turns occurs, by virtue of the phasechange when the length of the antenna exceeds one fourth of the wavelength and particularly when it nears the half-wave length, which occurswhen the antenna reaches approximately 11 m at the currently usedoperating frequency of 13.56 MHz.

This is why the purpose of the invention is to produce a spiral typetransmission and/or reception antenna in which there is no currentdissipation due to inter-turn capacitance regardless of the dimensionsof the antenna turns.

The object of the invention is thus an electromagnetic wave transmissionand/or reception antenna of the type featuring a flat spiral wire, saidspiral having at least two turns, this antenna being characterized inthat it includes at least one cut in the antenna wire for the purpose ofreducing the inter-turn capacitance.

The purposes, objects and characteristics of the invention will becomemore apparent from the following description when taken in conjunctionwith the accompanying drawings in which:

FIG. 1 represents a three-turn spiral antenna allowing implementation ofthe invention,

FIG. 2 represents the electronic circuit equivalent to the antennaillustrated in FIG. 1.

FIG. 3 represents the antenna shown in FIG. 1 in which the cut has beenmade,

FIG. 4 represents the electronic circuit equivalent to the antennaillustrated in FIG. 3,

FIG. 5 schematically represents the wires of the antenna with the cutoccurring in the parallel capacitance of the antenna portion located onone side of the cut,

FIG. 6 schematically represents the antenna wires with the cut occurringin the parallel capacitance of the antenna portion located on the otherside of the cut,

FIG. 7 schematically represents the antenna wires with the cut occurringin the series capacitance located between the two parts of the antenna,and

FIG. 8 represents the series circuit equivalent to the antennaillustrated in FIG. 3.

The antenna 10, shown in FIG. 1, can be used as a transmitter antenna ina contactless communication system where each user possesses a card (ora ticket) also equipped with an antenna. Electromagnetic signalstransmitted by the antenna of a reader such as the antenna 10 arecaptured by the antenna in the user's card which then retransmits otherelectromagnetic signals to antenna 10 granting the user access to acontrolled access zone.

As explained above, the antenna 10 may be relatively large and feature asignificant number of turns if a large operating volume is desired. Theantenna 10 may be represented by the electronic circuit in FIG. 2, theparallel capacitance C between turns becomes very high in relation tothe antenna's inductance L. If ω is the pulse used (ω=2πf), theimpedance due to the capacitance becomes much less large than theantenna inductance according to the formula$\frac{1}{C \cdot \omega} < {L \cdot \omega}$

At the very worst, the antenna itself is short-circuited by theinter-turn capacitance and hardly any current passes in the antenna. Asthe magnetic field emitted is proportional to the current running in theantenna, it is low and the result opposite that desired is achieved.

In order to offset this inconvenience, the parent idea behind theinvention is to make one or more cuts in the antenna wire. A cut such ascut 12 made in the antenna illustrated in FIG. 3, is in fact a definiteinterruption in the antenna wire of several millimeters and may reachseveral centimeters.

The electronic circuit equivalent to the antenna having a cut thusbecomes the circuit represented in FIG. 4 where the part located infront of the cut is equivalent to the inductance L1 in parallel with theinter-turn capacitance C1, and the part located after the cut isequivalent to an inductance L2 in parallel with the inter-turncapacitance C2, the two parts being linked by a series capacitance C3.

The capacitance values C1, C2 and C3 are due to capacitive couplingbetween certain antenna wires as illustrated in FIGS. 5, 6 and 7. Inthis manner, the parallel capacitance C1, is due to the capacitivecoupling between antenna wires 14 and 14′ and the parallel -capacitanceC2 is due to the capacitive coupling between wires 16′ and 16″, wires18′ and 18″ and wires 20′ and 20″. As far as the series capacitance C3is concerned, it is due to the capacitive coupling between wires 16 and16′, wires 18 and 18′, wires 20 and 20′ and wires 14′ and 14″.

Each cut made in the antenna thus enables Li-Ci pairs of lesser value oneach side of the cut than the L-C pair of the antenna with no cuts. Itcan thus initially be thought that as the number of cuts increases, theL-C pairs have low values which promote current in the inductanceelements. It is, in fact, judicious to provide a number of cutscorresponding to the antenna's series resonance, which corresponds tothe maximum current in the antenna and in the turns. The invention willbecome more apparent with the following example for the determination ofthe number of turns.

Firstly, one must understand that the purpose of the cuts made in theantenna is to significantly lower the values of L and C for each L-Cpair, located on either side of a cut. In this case, the impedance dueto the capacitance is distinctly greater than the inductance, i.e. inthe case of a single cut:${{L1}\quad\omega} < \frac{1}{{C1}\quad\omega}$

If ω1 is the pulse corresponding to the resonance of the cell L1, C1thus:${\omega\quad 1^{2}} = {{\frac{1}{L1C1}\quad{and}\quad\omega\quad 1} > \omega}$Consequently, this cell is equivalent to an inductance of value L1eq${L1eq} = \frac{Z1t}{j \cdot \omega}$${{where}\text{:}\quad\frac{1}{Z1t}} = {{\left( \frac{1}{j \cdot {L1} \cdot \omega} \right) + {{{jC} \cdot \omega}\quad{that}\quad{is}\quad\frac{1}{Z1t}}} = \frac{\left( {1 - {{L1} \cdot {C1} \cdot \omega^{2}}} \right)}{{j \cdot {L1}}\quad\omega}}$${{thus}\text{:}\quad{L1eq}} = {{\frac{L1}{\left( {1 - {{L1} \cdot {C1} \cdot \omega^{2}}} \right)}\quad{or}\quad{L1eq}} = \frac{L1}{\left\lbrack {1 - \left( \frac{\omega}{\omega\quad 1} \right)^{2}} \right\rbrack}}$

-   -   thus resulting in    -   L1eq>0 as ω1>ω

In the same manner, for cell L2, C2, we have${{L2}\quad\omega} < \frac{1}{{C2}\quad\omega}$

If ω2 is the pulse corresponding to the resonance of the cell L2, C2, wethus have:${\omega\quad 2^{2}} = {{\frac{1}{L2C2}\quad{and}\quad\omega\quad 2} > \omega}$

Cell L2, C2 is equivalent to an inductance of value L2eq:${L2eq} = {{\frac{L2}{\left( {1 - {{L2} \cdot {C2} \cdot}} \right)}\quad{or}\quad{L2eq}} = \frac{L2}{\left\lbrack {1 - \left( \frac{\omega}{\omega\quad 2} \right)^{2}} \right\rbrack}}$

-   -   thus resulting in: L2eq>0 as ω2>ω

Consequently, when the resonance frequency specific to each cell isdefinitely greater than the frequency of the current which passesthrough the antenna, the current is much greater in the turns than thatwhich flows through the inter-turn capacitors. The more this resonancefrequency specific to each cell increases, the more the currentincreases in the turns. This occurs when the number of cuts isincreased.

However, if the number of cuts is excessive, tuning between theantenna's equivalent inductance and the antenna's equivalent cutcapacitance may be impossible.

With N representing cuts equally distributed on the antenna, it can beinferred that the antenna was divided into N+1 identical cells, suchthat:Leq2=Leq2= . . . =Leq(N+1)

If Cci is the cut capacitance (or series capacitance) of cut i, thereare thus N identical cut capacitance values:Cc 1=Cc 2= . . . =CcN=Cc

If C is the inter-turn capacitance of each cell and Cant is theantenna's total inter-turn capacitance and by accepting an initialapproximation that the cut capacitance between two cells is equal to theinter-turn capacity of each cell, or Cc=C, we thus have:${Cc} = \frac{Cant}{{2N} + 1}$

It can thus be admitted that the electronic circuit equivalent to theantenna with N equally distributed cuts is that represented in FIG. 8,with: Leq = (N + 1) ⋅ Leq1${Ceq} = {\frac{Cc}{N} = \frac{Cant}{N\left( {{2N} + 1} \right)}}$

If ω2 is the pulse corresponding to the series resonance of the antennarepresented in FIG. 8, and if Lant is the total inductance of theantenna, then: $\begin{matrix}{{{{Leq} \cdot {Ceq} \cdot \omega^{2}} = {{{{1\left\lbrack {\left( {N + 1} \right) \cdot {Leq1} \cdot \frac{Cant}{\left( {{2 \cdot N} + 1} \right) \cdot N}} \right\rbrack} \cdot \omega}\quad r^{2}} = {1\quad{with}}}}\quad{{Leq} = {{{\left( {N + 1} \right) \cdot {Leq1}}\quad{and}\quad{Ceq}} = \frac{Cant}{\left( {{2 \cdot N} + 1} \right) \cdot N}}}{{{{Leq1} \cdot {Cant} \cdot \omega}\quad r^{2}} = \frac{\left\lbrack {\left( {{2 \cdot N} + 1} \right) \cdot N} \right\rbrack}{\left( {N + 1} \right)}}} & (1)\end{matrix}$

It has been seen that Leq1 may be written as: $\begin{matrix}{{Leq1} = \frac{L1}{\left( {1 - {{{L1} \cdot {C1} \cdot \omega}\quad r^{2}}} \right)}} \\{{Leq1} = {\frac{\left\lbrack \frac{Lant}{\left( {N + 1} \right)} \right\rbrack}{\left\lbrack {1 - {{\left\lbrack \frac{Lant}{\left( {N + 1} \right)} \right\rbrack \cdot \left( \frac{Cant}{{2 \cdot N} + 1} \right) \cdot \omega}\quad r^{2}}} \right\rbrack}\quad{with}}} \\{{L1} = {{\left\lbrack \frac{Lant}{\left( {N + 1} \right)} \right\rbrack\quad{and}\quad{C1}} = \left( \frac{Cant}{{2 \cdot N} + 1} \right)}} \\{{Leq1} = \frac{\left\lbrack {{Lant} \cdot \left( {{2 \cdot N} + 1} \right)} \right\rbrack}{{\left( {N + 1} \right) \cdot \left( {{2 \cdot N} + 1} \right)} - {{{Lant} \cdot {Cant} \cdot \omega}\quad r^{2}}}}\end{matrix}$

-   -   by using the relationship (1), N verifies:        $\left\lbrack \frac{\left\lbrack {{Lant} \cdot \left( {{2 \cdot N} + 1} \right)} \right\rbrack}{{\left( {N + 1} \right) \cdot \left( {{2 \cdot N} + 1} \right)} - {{{Lant} \cdot {Cant} \cdot \omega}\quad r^{2}}} \right\rbrack = \left\lbrack \frac{\left\lbrack {\left( {{2 \cdot N} + 1} \right) \cdot N} \right\rbrack}{{\left( {N + 1} \right) \cdot {Cant} \cdot \omega}\quad r^{2}} \right\rbrack$        such  that:  N ⋅ (N + 1) ⋅ (2 ⋅ N + 1) − 2 ⋅ N ⋅ Lant ⋅ Cant ⋅ ω  r² − Lant ⋅ Cant ⋅ ω  r² = 0        such  that:  N² + N − (Lant ⋅ Cant ⋅ ω  r²) = 0        ${{Thus}\text{:}\quad N} = {{\frac{\left( {{- 1} + \sqrt{\Delta}} \right)}{2}\quad{with}\quad\Delta} = \left( {1 + {{4 \cdot {Lant} \cdot {Cant} \cdot \omega}\quad r^{2}}} \right)}$        ${{Such}\quad{that}\text{:}\quad N} = \frac{\left\lbrack {{- 1} + \sqrt{\left( {1 + {{4 \cdot {Lant} \cdot {Cant} \cdot \omega}\quad r^{2}}} \right)}} \right\rbrack}{2}$

In this manner, if a transmitter antenna operating at 13.56 MHz isconsidered, the number of cuts to be made to obtain the series resonanceof the antenna can be calculated: N=3.444.

We can thus take N=3 or N=4 cuts.

With N=3, the proportion of current passing through the turns and theproportion of current dissipated by the inter-turn capacitance can becalculated:

-   -   an inter-turn capacitance value of        ${C1} = {{\frac{C}{{2 \cdot N} + 1}\quad{C1}} = {1.1017 \times 10^{- 11}}}$    -   an inductance value at pulse wr        ${L1} = {{\frac{L}{\left( {N + 1} \right)}\quad{L1}} = {8.64 \times 10^{- 6}}}$    -   the current in the turns is:        ${IL} = \frac{\left( \frac{1}{{{C1} \cdot \omega}\quad r} \right)}{\left\lbrack {\left( \frac{1}{{{C1} \cdot \omega}\quad r} \right) + {{{L1} \cdot \omega}\quad r}} \right\rbrack}$        IL = 0.611  (or  61%  of  the  total  current  in  the  antenna)    -   the current passing in the inter-turn capacitance is        ${IC} = \frac{\left( {{L1} \cdot \omega} \right)}{\left\lbrack {\left( \frac{1}{{{C1} \cdot \omega}\quad r} \right) + {{{L1} \cdot \omega}\quad r}} \right\rbrack}$        IC = 0.389  (or  39%  of  the  total  current  of  the  antenna)

1-7. (canceled)
 8. An electromagnetic transmission and/or receptionantenna comprising a wire formed in a flat spiral having at least twoturns, wherein said antenna includes at least one cut capable ofreducing an inter-turn capacitance.
 9. The antenna of claim 8, whereinsaid wire has a length at least equal to one fourth of the wavelength ofan electromagnetic wave.
 10. The antenna of claim 9, further comprisingat least one additional cut, and wherein the cuts are distributed insuch a way as to form equal portions of said wire on each side of thecut.
 11. The antenna of claim 10, comprising three equally distributedcuts.
 12. A contactless communication system, comprising a plurality ofcontactless cards each having an antenna, a card reader having theantenna of claim 8, such that the reader transmits electromagneticsignals to a contactless card in such a way as to be able to identifythe holder of said contactless card when the latter transmits returnidentification signals to said reader.
 13. The communication system ofclaim 12, wherein said contactless communication system is a system forgaining access to a controlled access zone.
 14. The communication systemof claim 13, wherein said electromagnetic signals have a frequency of13.56 Hz.